This invention relates to a device for controlling an electromechanical transducer delivering a mechanical quantity as its output corresponding to the magnitude of an electrical quantity applied as its input, and more particularly to a control circuit for accurately driving a proportional electromagnetic valve as instructed by an electrical instruction signal.
As the regulations on the toxic components in the exhaust gas of internal combustion engines of motor vehicles have been made more rigorous in recent years, there has arisen a demand for an exhaust gas purification system which can quickly and accurately remove the toxic components in the exhaust gas without appreciable aging and with high reliability. In an effort to comply with this demand, various exhaust gas purification systems have been proposed, and as a consequence of extensive researches and studies by the parties concerned, the exhaust gas purification systems have been steadily developed to the practically usable stage. Among the proposed and developed systems, the control systems including means for ensuring the reliability and reproducibility of control and capable of compensating the aging and other problems are considered most promising. A closed-loop control system controlling the air-fuel ratio in the carburetor utilizing a ternary catalyst, and an engine combustion control system utilizing a microcomputer for the air-fuel ratio control are examples of the promising control systems. In such a control system, an electromechanical transducer is essentially required which can quickly and accurately respond to an instruction signal applied from a control circuit. A variety of such electromechanical transducers are presently under investigation by the parties concerned and include:
(1) An electromechanical transducer (a proportional electromagnetic valve) delivering a mechanical quantity as its input corresponding to the magnitude of an electrical quantity applied as its input;
(2) an electromechanical transducer (an on-off electromagnetic valve) delivering a mechanical quantity as its output which takes either a minimum value or a maximum value depending on whether an electrical quantity applied as its input has a level higher or lower than a predetermined threshold level; and
(3) an electromechanical transducer in the form of a servomotor or a stepping motor.
The electromechanical transducer in (3) includes the problems of high cost, low control accuracy, slow response, etc. compared with those in (1) and (2), and investigations on the electromechanical transducers in (1) and (2) are presently extensively being done.
However, the electromechanical transducers in (1) and (2) include also their own peculiar problems. The electromechanical transducer in (1), or the proportional electromagnetic valve, includes the following problems:
(a) The proportional electromagnetic valve is the electromechanical transducer which delivers a mechanical quantity, that is, displacement or force as its output corresponding to the magnitude of an electrical quantity, that is, current supplied as its input. It is therefore necessary to subject an output signal of its control circuit to digital-analog (D-A) conversion when a digital means such as a microcomputer is used therewith.
(b) Hysteresis tends to occur in the mechanical quantity, and the accuracy of control tends to be reduced. The term "hysteresis in the mechanical quantity" is used herein to denote such a phenomenon that the position of the plunger of the electromagnetic valve responding to a constant current value supplied to the electromagnetic coil differs depending on whether the plunger makes an advancing stroke or a retracting stroke. It will thus be obvious that this hysteresis impairs the performance of the proportional electromagnetic valve.
(c) Means for counterbalancing the electromagnetic force, that is, a spring system is used to provide the mechanical quantity corresponding to the electrical quantity. Consequently, the proportional electromagnetic valve tends to be adversely affected by disturbance such as vibration externally impacted thereto.
On the other hand, the electromechanical transducer in (2), or the on-off electromagnetic valve, includes the following problems:
(d) The on-off electromagnetic valve is the electromechanical transducer which delivers a mechanical quantity, that is, displacement as its output which takes a minimum value or a maximum value depending on whether an electrical quantity applied as its input has a level higher or lower than a predetermined threshold level. That is, the on-off electromagnetic valve has such a property that it does not produce any mechanical displacement unless a predetermined electrical quantity is applied as its input. In order to derive the mechanical quantity corresponding to the electrical quantity, therefore, it is necessary to control the duty cycle of the electrical quantity applied to the on-off electromagnetic valve for directly controlling a pressure, for example, the differential pressure across the air jet, or to utilize a pressure medium, for example, the engine suction vacuum or Venturi vacuum as an actuating source and control the magnitude of this actuating source for indirectly controlling a pressure responsive means such as a diaphragm means which converts a pressure into a mechanical quantity such as displacement. Therefore, even when the electrical quantity applied to the on-off electromagnetic valve as its input exceeds the predetermined threshold level within a short period shorter than the minimum response time required for the mechanical quantity (displacement) to reach its maximum value from its minimum value, the on-off electromagnetic valve cannot mechanically follow the variation of the input within such a short period, and no variation occurs in the mechanical quantity. Thus, the mechanical quantity does not vary in the region in which the on-off period ratio, that is, the duty cycle of the electrical quantity input is small, and a dead zone of control results. This dead zone will be explained with reference to FIG. 1. Suppose that TB and TA represent the on-state period of a voltage V applied to the on-off electromagnetic valve, and one cycle of the applied voltage V respectively. Then, the stroke of the plunger of the electromagnetic valve is zero, that is, the plunger makes no displacement when the on-state period TB is short, or when the duty cycle is small. The dead zone appears in such a case.
(e) Due to the continual reciprocating movement of the means for transmitting the mechanical quantity, for example, the needle valve member between the minimum value and the maximum value of its full stroke, considerable material wear occurs at the parts such as the bearing, valve seat and stopper engaged and struck by the needle valve member, and a considerable crashing sound is also produced at the parts struck by the needle valve member.
(f) Since the duty cycle of the electrical quantity applied to the on-off electromagnetic valve is controlled to control the mechanical output of the valve between its minimum value and its maximum value, vibration or pressure ripple tends to occur in the final mechanical quantity, for example, the air jet negative pressure.
As pointed out above, both the proportional electromagnetic valve in (1) and the on-off electromagnetic valve in (2) have included their own peculiar problems, and it has been demanded to make further improvements to obviate these problems of the electromechanical transducers so that the transducers can be reliably used in motor vehicles for various control purposes.